Item Cast spell

When Hero used Item like Staff of Teleportion ?

it's Trigger Event Unit begin spell

how do I know the Item that the hero used to cast the spell ?

don't tell me to used Event Unit Used Item since I can't retrieve the target

I'm not sure you can. However, you can use both events. When the event "uses an item" happens, you store the used item into a variable, and when the "starts the effect of an ability" event happens, and the ability being cast is an item ability, then the item stored in the variable is your last used item.